1. Field of the Invention
The present invention relates to an image display and a method of driving the image display, more particularly to an image display which can reduce fluctuation in luminance due to parasitic capacitance.
2. Description of the Related Art
Conventionally, an image display with an organic light emitting diode which causes light emission through control of electric current has been proposed. Such an OLED has a function of emitting light via recombination of positive holes and electrons injected into a light emitting layer.
In such an image display, a thin film transistor (TFT), which is formed from, for example, amorphous silicon or polycrystalline silicon, and an organic light emitting diode constitutes each pixel. The luminance is controlled through setting of appropriate value of electric current for each pixel.
FIG. 17 is a diagram of a structure of a pixel circuit corresponding to one pixel in a conventional image display. In FIG. 17, the pixel circuit includes an organic light emitting diode OLED, an organic light emitting diode capacitor COLED, a driving transistor Td, a threshold voltage detecting transistor Tth, a supplementary capacitor Cs, a switching transistor T1, and a switching transistor T2.
The driving transistor Td serves to control an amount of electric current flowing through the organic light emitting diode OLED according to potential difference applied between a gate electrode and a source electrode. The threshold voltage detecting transistor Tth, on attaining an ON state, electrically connects the gate electrode and the drain electrode of the driving transistor Td, to cause electric current to flow from the gate electrode to the drain electrode of the driving transistor Td until the potential difference between the gate electrode and the source electrode of the driving transistor Td reaches a threshold voltage Vth of the driving transistor Td. Thus the threshold voltage detecting transistor Tth serves to detect a threshold voltage Vth of the driving transistor Td.
The organic light emitting diode OLED has a characteristic of emitting light when a potential difference equal to or higher than a threshold voltage (i.e., potential difference between an anode and a cathode) is generated to cause electric current to flow. Specifically, the organic light emitting diode OLED has at least an anode layer and a cathode layer formed from Aluminum (Al), Copper (Cu), Indium Tin Oxide (ITO), or the like, and a light emitting layer formed from an organic material such as pthalcyanine, tris-aluminum complex, benzoquinolinolato, and beryllium complex, between the anode layer and the cathode layer. The organic light emitting diode OLED emits light by recombining the positive hole and the electron injected to the light emitting layer. The organic light emitting diode capacitor COLED is an equivalent representation of capacitance of the organic light emitting diode OLED.
The driving transistor Td, the threshold voltage detecting transistor Tth, the switching transistor T1, and the switching transistor T2 are, for example, thin film transistors. Though the drawings referred to below do not clearly show channel type of each thin film transistor, i.e., whether the TFT is n-type or p-type, the TFT is one of n-type and p-type and should be interpreted in accordance with the description below.
A power source line 10 supplies power to the driving transistor Td and the switching transistor T2. A Tth control line 11 supplies a signal for controlling the threshold voltage detecting transistor Tth. A merge line 12 supplies a signal for controlling the switching transistor T2. A scan line 13 supplies a signal for controlling the switching transistor T1. An image data line 14 supplies image data.
The pixel circuit with the above-described structure operates through four periods, i.e., a preparing period, a threshold voltage detecting period, a writing period, and a light emitting period. In the preparing period, a positive potential of a predetermined level (Vp, Vp>0) is applied to the power source line 10 to render the threshold voltage detecting transistor Tth OFF, the switching transistor T1 OFF, the driving transistor Td ON, and the switching transistor T2 ON. As a result, electric current flows sequentially from the power source line 10, the driving transistor Td, to the organic light emitting diode capacitor COLED, thereby accumulating electric charges in the organic light emitting diode capacitor COLED.
In the threshold voltage detecting period, zero potential is applied to the power source line 10, to render the threshold voltage detecting transistor Tth ON, and to connect the gate and the drain of the driving transistor Td. Thus, the electric charges accumulated in the supplementary capacitor Cs and the organic light emitting diode capacitor COLED are discharged to cause electric current to flow from the driving transistor Td to the power source line 10. When the potential difference between the gate and the drain of the driving transistor Td reaches the level of the threshold voltage Vth corresponding to driving threshold of the driving transistor Td, the driving transistor Td is turned OFF.
In the writing period, the power source line 10 maintains zero potential, while the switching transistor T1 is turned ON, and the switching transistor T2 is turned OFF, to discharge the electric charges accumulated in the organic light emitting diode capacitor COLED. As a result, the electric current flows from the organic light emitting diode capacitor COLED, the threshold voltage detecting transistor Tth, and to the supplementary capacitor Cs, to accumulate electric charges in the supplementary capacitor Cs. In other words, the electric charges accumulated in the organic light emitting diode capacitor COLED moves to the supplementary capacitor Cs.
In the light emitting period, a negative potential of a predetermined level (−VDD, VDD>0) is applied to the power source line 10, to render the driving transistor Td ON, the threshold voltage detecting transistor Tth OFF, and the switching transistor T1 OFF. As a result, an electric current flows from the organic light emitting diode OLED, the driving transistor Td, to the power source line 10, thereby causing the organic light emitting diode OLED to emit light.
For more detailed information on conventional technique, see, for example, S. Ono et al., Proceedings of IDW, '03, 255 (2003).
The conventional image display as shown in FIG. 18, includes a parasitic capacitor CgdTd and a parasitic capacitor CgsTd in the vicinity of the gate of the driving transistor Td, and a parasitic capacitor CgdTth and a parasitic capacitor CgsTth in the vicinity of the gate of the threshold voltage detecting transistor Tth.
The parasitic capacitors are known to cause fluctuation in luminance of the organic light emitting diode OLED. Hence, technique to effectively minimize negative effect of the parasitic capacitors is conventionally desired.